α1‐adrenoceptor blockade and/or 5‐HT1A agonism during treadmill exercise in dogs

Abstract

1. The hypotensive effects of alpha 1-adrenergic blockade and/or stimulation of central nervous 5-HT1A receptors were studied using drugs with different affinity for central nervous 5-HT1A and peripheral alpha 1-adrenoceptors. Urapidil, 5-methylurapidil, flesinoxan and 8-OH-DPAT were compared under states of different activation of the autonomic nervous system, i.e. at rest and during graded treadmill exercise. 2. The rank order of hypotensive potency as derived from the most extensive decrease in resting diastolic arterial blood pressure was urapidil greater than 5-methylurapidil greater than flesinoxan much greater than 8-OH-DPAT. 3. The reflex increase in heart rate due to the decrease in arterial blood pressure at rest was suppressed after 0.1 mumol kg-1 flesinoxan. 4. The reflex increase in heart rate due to the decrease in arterial blood pressure at rest was less accentuated after high doses of urapidil and 5-methylurapidil. 5. During exercise both 5HT1A receptor agonists, flesinoxan and 8-OH-DPAT, decreased sympathetic tone. 6. The combined effects of alpha 1-adrenoceptor blockade and 5-HT1A receptor stimulation (urapidil and 5-methylurapidil) result in distinct decreases in blood pressure and slight suppression of reflex tachycardia at rest after high doses. Stimulation of 5-HT1A receptors alone (flesinoxan) suppresses reflex tachycardia by modulation of baroreceptor reflex and at high dose also diminishes exercise-induced increase in sympathetic tone.